1. Field of the Invention
The invention is related generally to the area of circuit designs, particularly to an AC-DC (Alternating Current to Direct Current) flyback converter and a loop compensation method thereof in discontinuous conduction mode (DCM).
2. Description of Related Art
In the electronic industry, higher performance of high-frequency switching power supply is always demanded. In the prior art, flyback converters are widely used for various low power supplies and power adapters due to simple circuit structure and inherent low cost.
These flyback converters are also referred to as buck-boost converters. An AC-DC flyback converter is derived from a buck-boost converter by adding an isolation transformer. The AC-DC flyback converters are configured for converting the alternating current into the direct current and are widely used in chargers for various portable devices and power adapters. Generally, the AC-DC flyback converters have two working modes, one is discontinuous conduction mode (DCM) and the other is continuous conduction mode (CCM). The AC-DC flyback converters work in the DCM mode for most applications. Thus, the AC-DC flyback converters in the present invention work in the DCM mode except for special cases.
FIG. 1 shows an exemplary circuit diagram of a conventional AC-DC flyback converter 100. The AC-DC flyback converter 100 includes a transformer T which includes a primary winding, a secondary winding, and an auxiliary winding. A controller 102 and a power switch 101 are provided one the side of the primary winding of the voltage transformer T. A rectifier diode D, an output capacitor Gout, an equivalent series resistor Resr of the output capacitor Gout, and a loading resistor Rload are provided on the side of the secondary winding of the voltage transformer T. Two divider resistors Rfb1 and Rfb2 are provided at the side of the auxiliary winding of the voltage transformer T.
When the power switch 101 is turned on upon a pulse width modulation signal sent from the controller 102, an electric current runs through the primary winding of the voltage transformer T, where the voltage polarity of the upper end of the primary winding is positive, and the voltage polarity of the lower end of the primary winding is negative. However, the conducted voltage polarity of the upper end of the secondary winding is negative, the conducted voltage polarity of the lower end of the secondary winding is positive, and then the rectifier diode D is reverse biased to be turned off, thereby the power energy is stored in the voltage transformer T in the form of magnetic energy. When the power switch 101 is turned off, the voltage polarities of the two ends of the primary winding reverse, i.e., the polarity of the upper end of the primary winding is negative, and the polarity of the lower end of the primary winding is positive. The voltage polarities of the secondary winding also reverse, i.e., the polarity of the upper end of the secondary winding is positive, the polarity of the lower end of the secondary winding is negative, the rectifier diode D is turned on, and then the energy stored in the voltage transformer is delivered to the loading resistor Rload. The conversion from AC to DC is achieved.
Furthermore, the divider resistors Rfb1 and Rf at the side of the auxiliary winding are provided for detecting the output voltage and providing a feedback voltage to the controller 102. Under lower power application environment in the prior art, the AC-DC flyback converter generally works on the discontinuous conduction mode (DCM), thereby the output voltage can be adjusted accurately, and some peripheral components with low cost and small size can use to reduce the production cost.
However, equivalent inductance L of the transformer T used in the AC-DC flyback converter 100 is usually larger (e.g., L=1.5 mH). A working frequency of the AC-DC flyback converter is usually from 40 KHz to 200 KHz in order to avoid electromagnetic interference (EMI). In real applications, the bandwidth of the system loop is required to be among ⅕ to 1/10 of the working frequency in order to keep stability of the feedback loop and depress noise interference. In other words, a high performance (stable and low noise) AC-DC flyback converter could be obtained if there is an enough phase margin in the lower unity gain bandwidth of the system loop. However, the AC-DC flyback converter in the prior art usually has a larger bandwidth of the system loop and a lower system stability.